1. Field of the Invention
This invention relates to methods and apparatus for producing n-type semiconducting diamond, and more particularly to such methods which utilize a uniformly distributed concentration of doping lithium through neutron transmutation.
2. Background Art
The present invention was developed to fill a need for a semiconducting diamond material with n-type conductivity. Semiconducting diamond has numerous attributes which are attractive for high-frequency, high-power semiconductor applications. These properties include a high electrical field breakdown voltage, elevated temperature stability, high electron and hole mobilities, high thermal conductivity, and excellent resistance to radiation.
A number of techniques for making diamond have been developed, such as high pressure, high temperature synthesis, shockwave synthesis, and chemical vapor deposition (including direct-current plasma chemical vapor deposition, microwave plasma chemical vapor deposition, and heated filament-assisted chemical vapor deposition).
Semiconducting synthetic or natural diamonds are mostly prepared or found as p-type materials, with boron atoms being the most common impurity. M. W. Geiss, D. D. Rathman, D. J. Ehrlich, R. A. Murphy, W. T. Lindley, "High-Temperature Point-Contact Transistors and Schottky Diodes Formed on Synthetic Boron-Doped Diamond," IEEE Electron Device Letters, Vol. EDL-8, No. 8, (August 1987), pp. 341-343, discloses the formation of point-contact transistors on synthetic boron-doped diamond. The diamond used in formation of the point-contact transistor is a diamond single crystal produced using a high-temperature-high-pressure process. H. Shiomi, y. Nishibayashi, N. Fujimore, "Electrical Characteristics of Metal Contacts to Boron-Doped Diamond Epitaxial Films," Japanese Journal of Applied Physics, Vol. 28, No. 5, (May 1989), pp.758-762, teaches the production of a planar field effect transistor device based on a diamond film. The device disclosed comprises a single crystal diamond substrate on which a single crystal epitaxial layer of boron doped diamond is deposited, thereby producing a p-type semiconductor layer. Titanium (Ti) source and drain contacts, as well as an aluminum (Al) gate Schottky contact are deposited on the diamond film.
Type IIb diamonds are described, in A. S. Vishnevskil, A. G. Gontar, "Electrical Conductivity of Heavily Doped P-Type Diamond," Soviet Physics-Semiconductor, Vol. 15(6), (June 1981), pp. 659-661; A. S. Vishnevskil, A. G. Gontar, "Electrical Conductivity of Synthetic Diamond Crystals," Soviet Physics-Semiconductor, Vol. 11 (1), (October 1977), pp. 1186-1187; G. N. Bezrukov, L. S. Smirnov, "Some Electrical and Optical Properties of Synthetic Semiconducting Diamonds Doped With Boron," Soviet Physics-Semiconductor, Vol. 4(4), (October 1970), pp. 587-590; J. J. Hauser, J. R. Patel, "Hopping Conductivity in C-Implanted Amorphous Diamond, or How to Ruin a Perfectly Good Diamond," Solid State Communications, Vol. 18, (1976), pp. 789-790; I. G. Austin, R. Wolfe, "Electrical and Optical Properties of a Semiconducting Diamond," Proc. Phys. Soc., (1956), pp. 329-338; P. T. Wedepohl, "Electrical and Optical Properties of Type IIb Diamonds," Proc. Phys. Soc., Vol. LXX, No. 2B, (1957), pp. 177-185; A. T. Collins, A. W. S. Williams, "The Nature of the Acceptor Centre in Semiconducting Diamond," J. Phys. C: Solid St. Phys., Vol. 4, (1971), pp. 1789-1800; and V. S. Vavilov, "Ion Implantation into Diamond," Radiation Effects, Vol. 37, (1978), pp. 229-236.
Impurities such as lithium which produce n-type diamond have been incorporated into previously formed diamond crystal lattices by ion implantation methods. See "Electrical Properties of Diamond Doped by Implantation of Lithium Ions," V. S. Vavilov, E. A. Konorova, E. B. Stepanova, and E. M. Trukhan, Soviet Physics-Semiconductors, Vol. 13(6), (1979), pp. 635-638. However, this method produces only thin layers of n-type diamond, the thickness of which is limited by the energy of accelerated ions that are not infinite. Moreover, the incorporation of n-type impurities into diamond crystal lattices by ion implantation causes a severely damaged surface layer due to graphitization of the diamond which cannot be removed by annealing. Doping with lithium by this method to concentrations higher than 10.sup.15 cm.sup.-3 also leads to graphitization. The implanted crystal must then be heat treated to electronically activate the implanted impurity. The severely damaged layer resulting from ion implantation and the inhomogeneity and significant concentration gradients of the implanted ion across the implanted film thickness render the n-type diamond unsuitable for semiconductor applications.
Another method for producing n-type diamond involves the formation of p-type diamond thin film by adding a boron compound such as diborane to the raw material gas for microwave plasma chemical vapor deposition and then converting boron-10 to lithium-7 by neutron irradiation. However, the chemical vapor deposition method limits the depth of the semiconducting substrate to approximately 100 microns. Moreover, the material is unsuitable for semiconductor materials because of radioactive isotopes created during neutron irradiation.
Other attempts to make an n-type semiconducting diamond are described in V. S. Vavilov, E. A. Konorova, "Conductivity of Diamond Doped by Implantation of Phosphorus Ions," Soviet Physics-Semiconductors, Vol. 9(8), (1976), pp. 962-964; V. S. Vavilov, E. A. Konorova, "Implantation of Antimony Ions Into Diamond," Soviet Physics-Semiconductors, Vol. 6(12), (1972), pp. 1998-2002; Jean-Francois Morhange, "Study of Defects Introduced by Ion Implantation in Diamond," Japanese Journal of Applied Physics, Vol. 14(4), (1975), pp. 544-548; Tsai et al, "Diamond MESFET Using Ultrashallow RTP Boron Doping," IEEE Electron Device Letters, Vol. 12, No. 4, (April 1991), pp. 157-159 and Gildenblat, et al., "High-Temperature Thin-Film Diamond Field-Effect Transistor Fabricated Using a Selective Growth Method," IEEE Electron Device Letters, Vol. 12, No. 2, (Feb. 1991), pp. 37-39.
There remains a need to provide a more satisfactory solution for producing a diamond semiconductor material with n-type conductivity.